The present invention relates generally to the field of lithium-ion batteries and methods of making such batteries.
Lithium-ion batteries or cells include one or more positive electrodes, one or more negative electrodes, and an electrolyte provided within a case or housing. Separators made from a porous polymer or other suitable material may also be provided intermediate or between the positive and negative electrodes to prevent direct contact between adjacent electrodes. The positive electrode includes a current collector (e.g., aluminum such as an aluminum foil) having an active material provided thereon (e.g., LiCoO2), and the negative electrode includes a current collector (e.g., copper such as a copper foil) having an active material (e.g., a carbonaceous material such as graphite) provided thereon. The active materials for the positive and negative electrodes may be provided on one or both sides of the current collectors.
During charging and discharging of the battery, lithium ions move between the positive electrode and the negative electrode. For example, lithium ions flow from the negative electrode to the positive electrode during discharging of the battery, and in the opposite direction during charging.
One issue associated with conventional secondary (i.e., rechargeable) lithium-ion batteries is lithium plating. Lithium plating, which is a well-known phenomenon characterized by a buildup of lithium metal on the negative electrode, may occur when the potential of the negative electrode drops to 0.0 volts (V) versus Li/Li+. Lithium plating can result in decreased battery capacity, since the buildup of lithium on the negative electrode decreases the amount of cyclable lithium available in the battery. Internal battery shorts may also result from lithium plating, for example, when the buildup of lithium becomes so significant that dendrites form on the negative electrode and extend to and make contact with an adjacent positive electrode.
The issue of lithium plating is particularly problematic in batteries having negative active materials that exhibit relatively low potentials versus Li/Li+. Carbon-based active materials commonly used in lithium secondary batteries, for example, have an average potential of approximately 0.1 V versus Li/Li+.
To reduce the likelihood of lithium plating, battery manufacturers typically provide excess negative electrode capacity to balance the positive electrode capacity, particularly at relatively high-current regions of the negative electrodes (e.g., edges of the electrodes). The additional negative active material provides additional intercalation sites for the cyclable lithium originating with the positive active material.
One method of providing additional negative active material is to utilize negative electrodes that have different (e.g., larger) physical dimensions than the positive electrodes. Depending on the type of battery configuration involved (e.g., wound electrode, flat plate electrode, Z-fold electrode, etc.), the manner in which the dimensions of the negative electrode differ from that of the positive electrode may vary.
For example, in a flat plate lithium battery that includes a plurality of positive and negative electrodes, lithium plating may occur at relatively high current density regions (e.g., near the four edges of the negative electrodes). To compensate for this tendency, conventional batteries are designed such that the negative electrodes (or the active material provided thereon) extend beyond the ends of the positive electrodes (or the active material provided thereon) in all directions by an amount sufficient to compensate for variations in the winding/assembly process (e.g., by an amount up to 1 millimeter or more).
FIGS. 1-2 illustrate a portion of a battery 100 that includes a plurality of negative electrodes 110 (which include a current collector 112 and active material 114 provided thereon), a plurality of positive electrodes 120 (which include a current collector 122 and active material 124 provided thereon), and separators 130 and electrolyte between the positive and negative electrodes. Although shown as being coextensive with the active material 114 in FIG. 1, the separator(s) may be provided such that it extends beyond the edge of the adjacent electrode to prevent shorting between adjacent electrodes (i.e., the separator may extend above and below the edge of the active material 114 shown in FIG. 1). The portion of the active material 114 that extends beyond the positive electrodes 120 is thus used to compensate for the tendency to plate lithium. The extent to which the negative electrodes extend beyond the edges of the positive electrodes (shown by arrows A-A and B-B in FIG. 2) may be up to one millimeter (mm) or more.
A similar design rule is used in the context of wound (e.g., jellyroll style) lithium batteries, as shown in FIGS. 3-4. As illustrated, a wound electrode battery 200 includes a negative electrode 210 (which includes a current collector 212 and active material 214 provided thereon) and a positive electrode 220 (which includes a current collector 222 and active material 224 provided thereon). Separators 230 are provided between the positive and negative electrodes. As illustrated in FIGS. 3-4, the top and bottom edges of the negative electrode 210 extend beyond the edges of the positive electrode 220 by a distance C that may be up to one millimeter or more. Additionally, because a leading edge 211 of the negative electrode is also a potential location where plating might occur, the leading edge 211 of the negative electrode 210 extends a distance D (e.g., up to one millimeter or more) beyond a leading edge 221 of the positive electrode 220.
In cases where an accordion-style fold (also referred to as a “Z fold” or a “zigzag” fold) is used for the electrodes, multiple design rules may be employed to mitigate lithium plating. For example, the top and bottom edges of the negative electrodes may extend beyond those of the positive electrodes in a manner similar to that described above with respect to wound electrodes. Accordian-style folding of the electrodes also provides an additional area of potential concern, however. FIG. 5 illustrates a battery 300 having an electrode set 302 that is folded accordion-style. The electrode set 302 includes a negative electrode 310 (which includes a current collector 312 and active material 314 provided thereon), a separator 330, and a positive electrode 320 (which includes a current collector 322 and active material 324 provided thereon). Because of the manner in which the electrode set 302 is folded, at certain localized areas within the battery 300, there will be a greater amount of positive active material than negative active material. One such area is shown in FIG. 5, where the positive electrode 320 is outside the negative electrode 310 at a fold. Because there is more positive active material in this region than negative active material (owing to the larger radius, and hence greater surface area, of the positive electrode in this area), the negative active material would be unable to take in all of the cyclable lithium provided by the positive active material in this area unless steps are taken to mitigate against such a circumstance. As shown in FIG. 5, one possible solution is to mask the positive current collector 322 such that no positive active material 324 is provided in this region (denoted with reference numeral 326). One difficulty with such a solution is that it is relatively complicated to accurately mask regions on the electrodes to ensure that they will line up appropriately at locations of the folds.
Another potential solution for accordion-style folded electrodes is shown in FIG. 6, which shows a portion of a battery 400 having a folded negative electrode 410 (which includes a current collector 412 and active material 414 provided thereon) with positive electrode plates 420 (which includes a current collector 422 and active material 424 provided thereon). Separators 430 are provided between the positive and negative electrodes. By providing positive plates 420 interspersed within the folded electrode arrangement, there is additional negative active material near each of the rounded folded portions of the negative electrode 410 to take in any excess lithium from the positive active material 424.
Each of the various configurations described above suffers from various drawbacks. For example, each of the above-described configurations utilizes a larger negative electrode as compared to the positive electrode (with an associated amount of additional active material provided thereon), which increases the overall size of the battery and results in an increased materials cost. Additionally, each of these configurations requires careful alignment of the various components to ensure that the negative active material is properly positioned such that it will absorb excess lithium from the positive material, which complicates manufacturing processes and introduces increased labor and equipment costs. In certain cases, negative electrodes may be made even larger than are necessary to absorb the excess lithium in order to compensate for potential variability in manufacturing processes which may result in slight misalignments of the electrodes. Further still, the use of excess electrode area and active material results in batteries having lower energy density than might otherwise be obtained, since the excess electrode area does not fully contribute to the capacity of the battery.